You might think that worms can't see. But some worms, such as the above
Dugesia
flatworm, do have eyespots. These are not exactly like your eyes, however. Eyespots can only detect light in their environment.

Flatworms

The word "worm" is not very scientific. But it is a word that informally describes animals (usually invertebrates) that have long bodies with no arms or legs. (Snakes are vertebrates, so they are not usually described as worms.) Worms show bilateral symmetry, meaning that the right side of their bodies is a mirror of the left.

One type of worm is the
flatworm
. Worms in the phylum
Platyhelminthes
are called flatworms because they have flattened bodies. There are more than 18,500 known species of flatworms.

Features of Flatworms

Flatworms have no true body cavity, but they do have bilateral symmetry.

Flatworms have an incomplete digestive system. This means that the digestive tract has only one opening. Digestion takes place in the gastrovascular cavity.

Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their body. Oxygen enters the pores by diffusion.

There are no blood vessels in the flatworms. Their
gastrovascular cavity
helps distribute nutrients throughout the body.

Flatworms have a ladder-like nervous system; two interconnected parallel nerve cords run the length of the body.

Most flatworms have a distinct head region that includes nerve cells and sensory organs, such as eyespots. The development of a head region, called
cephalization
, evolved at the same time as bilateral symmetry in animals.

Marine flatworms can be brightly colored, such as this one from the class Turbellaria. These worms are mostly carnivores or scavengers.

Flatworms in the Environment

Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms and rotting matter. These types of flatworms include marine flatworms and freshwater flatworms, such as
Dugesia
.

Other types of flatworms are parasitic. That means they live inside another organism, called a host, in order to get the food and energy they need. For example,
tapeworms
have a head-like area with tiny hooks and suckers (known as the
scolex
) that help the worm attach to the intestines of an animal host (
Figure
below
). There are over 11,000 species of parasitic flatworms.

Tapeworms are parasitic flatworms that live in the intestines of their hosts. They can be very long (
left
). Tapeworms attach to the intestinal wall with a head region that has hooks and suckers (
right
).

Vocabulary

cephalization
: Concentration of the sense organs and nervous tissue in a head region.

What is internal fertilization as opposed to external fertilization? Considering that there is an energetic cost to making gametes, what advantage can internal fertilization have over external fertilization?